1. Field of the Invention
The present invention relates to a focus detection system which is incorporated in optical equipment such as an SLR camera.
2. Description of the Related Art
In a conventional phase difference type focus detection system incorporated in an SLR camera system to serve an autofocus (AF) system thereof, a pair of object images obtained by pupil division are projected onto a pair of areas on a line sensor (line sensor array) to be converted into a pair of electric charges (electrical signals) by the line sensor, and the pair of electric charges are converted as a pair of image signals, respectively. Thereafter, a phase difference between the pair of object images formed on the line sensor is determined according to the pair of image signals and a focus state (defocus amount) is determined based on the phase difference.
In the case where such a focus detection system is a multipoint focus detection system capable of focusing on a plurality of focus detection zones, a monitor sensor (monitor sensor array) is installed on each of a plurality of line sensors or each of a plurality of zones on each line sensor which respectively correspond to the plurality of focus detecting zones, and the integral operation terminating operation and control on an amplification factor (gain) for amplification of image signals are carried out in accordance with one of the plurality of monitor sensors which has received an object image the brightness of which is the greatest. This type of multipoint focus detection system is disclosed in, e.g., U.S. Pat. No. 6,198,506.
Conventionally, in dark conditions, the integral operation is completed after a lapse of a predetermined maximum integral time, and image signals are amplified by an amplifier. If image signals having an adequate level are not obtained because of a low object brightness, the integration termination value is reduced or the amplification factor of the amplifier is increased to perform the integral operation again.
Due to the tendency of the number of focus detection zones being increased and further miniaturization of an optical system of the autofocus (AF) system in recent years, the amount of light which illuminates the light receiving surface of each line sensor has in turn being getting lower, i.e., there has been a tendency for an object image projected onto each line sensor to become dark. Due to this tendency, the amplification factor of the amplifier needs to be increased to obtain an appropriate integral amount.